Liquid crystal molecules of a Thin Film Transistor Liquid Crystal Display (TFT-LCD) cannot be maintained under a certain constant voltage all the time, otherwise, the liquid crystal molecules degrade and cannot rotate in response to change of electric field to form different gray scales even though the certain constant voltage is canceled. Therefore, the voltage must be inverted every a certain period of time so that degradation of the characteristics of the liquid crystal molecules can be avoided.
In a manufacturing process of the TFT-LCD, a parallel-plate capacitor formed between a pixel electrode and a gate scanning line or a common electrode is employed as a storage capacitor Cs. The Cs formed between the pixel electrode and the common electrode is widely used because of its wide applicability. In a case that voltage of the common electrode is fixed all the time, positive polarity can be obtained as long as voltage of the pixel electrodes is provided to be greater than that of the common electrode, and negative polarity can be obtained as long as the voltage of the pixel electrodes is provided to be smaller than that of the common electrode. Therefore, the case that the voltage of the common electrode fixed is applicable to polarity inversion of various panels.